In heat-sensitive stencil printing, a stencil sheet with a thermoplastic resin film bonded to an ink-permeable porous support is used. The original image read by an optical sensor is sent as digital signals to a thermal head, and the heat of the thermal head melts the thermoplastic resin film, to perforate it, and printing ink exudes from the porous support through the holes formed by the perforation.
Conventionally known heat-sensitive stencil sheets have a structure in which a porous support such as tissue paper, nonwoven fabric or woven fabric formed by natural fibers, chemical fibers and/or synthetic fibers is bonded by an adhesive onto a thermoplastic resin film such as an acrylonitrile based film, polyester based film or vinylidene chloride based film (e.g., JP-A-51-002512, JP-A-51-002513, JP-A-57-182495, etc.).
In recent years, heat-sensitive stencil printings are improved by increasing the density of heating elements of the thermal head or reducing the energy necessary for the thermal head to precisely reproduce original prints, such as photographs, and shortening the stencil sheet making time, etc. In this connection, a stencil sheet higher in perforation sensitivity is in demand. On the other hand, a stencil sheet excellent in printing durability which is not deformed or broken even after printing a large number of sheets is in demand.
However, the conventional heat-sensitive stencil sheets present problems in that if the energy necessary for the thermal head is reduced, the film is perforated only insufficiently and forms white spots on a black solid area or makes fine characters blurred, and in that mass printing causes the stencil sheet to be wrinkled, or to be delaminated into the film and the support, or to be broken. The causes for the poor printability and printing durability of the conventional stencil sheets are considered to be that adhesive used for bonding the film and the porous support inhibits the film perforation and ink permeation, and that the water, organic solvent, etc. contained in the ink act on the adhesive, to lower its bonding strength.
To overcome the disadvantages of these conventional stencil sheets, various proposals have been made. For example, JP-A-58-147396 and JP-A-04-232790 propose to keep the amount of adhesive used as small as possible. Furthermore, as a method to avoid the use of an adhesive, JP-A-04-212891 proposes a heat-sensitive stencil sheet obtained by scattering and thermally bonding synthetic fibers onto one surface of a thermoplastic resin film. However, these methods were found to provide insufficient bonding strength, and to present a problem in that if it is attempted to obtain a sufficient bonding strength, the film is poorly oriented, to be insufficiently perforated, making it difficult to form a stencil as accurate as the original. Moreover, JP-A-06-305273 and JP-A-07-186565 disclose a stencil sheet obtained by thermally bonding a polyester film and polyester fibers respectively not stretched, and co-stretching them. The stencil sheet has sufficient bonding strength between the film and the fibrous support without using any adhesive, but does not have sufficiently good performance characteristics to obtain the highly sensitive and highly precise prints required in recent years.
The present invention addresses the above problems of known stencil sheets and seeks to provide a heat-sensitive stencil sheet excellent in film perforation sensitivity and printing durability.